Comparative performance of selected bond coats in advanced thermal barriercoating systems

An investigation was carried out to determine the comparative performance o f selected bond coats representing the diffusion aluminides and overlays in thermal barrier coating systems. Emphasis was placed upon oxidation behavi or, thermal stability, and failure mechanism. Isothermal oxidation tests we re carried out at temperatures in the range of 1000 degrees C to 1150 degre es C. Scanning electron microscopy combined with energy dispersive x-ray sp ectroscopy, x-ray diffraction, and transmission electron microscopy were us ed to characterize the coating microstructure. Among the bond coats examine d, overlays exhibited the best performance followed by Pt-aluminides and si mple alunimides for a given alloy substrate. However, for all types of bond coats, failure of the coating system occurred by decohesion of the oxide s cale at the oxide-bond coat interface. All bond coats examined were found t o be degraded by oxidation and interdiffusion with the alloy substrate perm itting the formation of non-protective oxide scale near the bond coat surfa ce. Platinum as well as active elements such as Hf and Y were identified as key elements in improving the performance of thermal barrier coating syste ms. (C) 2000 Kluwer Academic Publishers.